Modem electronics equipment that includes components such as integrated circuits typically employ interconnection devices and techniques for electrically interconnecting printed circuit boards or other components. Interconnection between the conductive pads or leads of a component to the conductive pads or traces of a circuit can be accomplished in a number of ways, such as with solder. In instances where the removal, replacement, or testing of components or a device is necessary during the lifetime of the device, some suitable electrical connector or disconnect is often used.
A "land grid array" (LGA) is an example of such a connection, in which the two circuit elements to be connected each have a plurality of contact points, arranged in a line or two-dimensional array. An array of interconnection elements, known as an "interposer", is placed between the two arrays to be connected, and provides the electrical connection between the contact points or pads.
LGA interposers described in the prior art are typically conductive buttons made of solid metal springs or metallized particles inserted in an array of appropriately-located holes in a sheet of insulating material. Another prior art conductive button is made entirely of randomly wound and compressed mesh of solid wires.
Interposers also are known that instead of conductive buttons or the like comprise a continuous dielectric film, typically an elastomer, that may contain uniformly distributed or localized metallic wires, particles or flakes or metallized polymer particles, called an "anisotropically conducting" film. In such an array, the conducting elements form so dense a mesh that so that they are not in register with the conducting pads of either circuit to be connected.
When conductive structures embedded in a polymer matrix is used, bending, buckling, and compression of such structures is said to be "on elastic foundation." (S. Timoshenko, Strength of Materials II, VanNostrand, Princeton, 1956, and S. Timoshenko and J. M. Gere, Theory of Elastic Stability, McGraw-Hill, New York, 1961).
A deficiency common to many such prior art interposers is that they may not provide good electrical conduction paths while maintaining desirably low overall contact forces. In other words, the mating surfaces of the interposer can exert undesirably high contact forces to produce the desired local contact force necessary for good electrical contact at the conducting elements disposed within the conductive button, film, or carrier.
Dozier, U.S. Pat. No. 5,473,510, "Land Grid Array Package/Circuit Board Assemblies and Methods for Constructing the Same", shows a land grid array for joining electrical components using compressible socket conductor pins arrayed in sockets, an arrangement that is dependent on the socket thickness to limit the compressibility when torque is applied to screws that join the assembly. PA0 Mroczkowski, et. al, U.S. Pat. No. 5,308,252, "Interposer Connector and Contact Element Therefor", discloses an electrical contact design for an interposer. The individual contact elements for the interposer are "double-S" shaped pieces of metal which compress to make electrical contact. PA0 Grabbe, U.S. Pat. No. 5,653,598, "Electrical Contact with Reduced Self-Inductance" describes a land grid array connector that includes an insert having coil springs for electrically connecting contact pads of circuit components. In this spring interposer, the pitch is 70 mil, and it is very difficult to scale down. Also it needs a high insertion force, and is of high cost because the parts for each interposer connection are individually made and assembled. Another description of a similar system to this patent was reported by the inventor in 1991 in the AMP Journal of Technology, vol. 1, pp.80-90, in an article entitled "High Density Electronic Connector (Micro Interposer) for High Speed Digital Applications." PA0 Grabbe, U.S. Pat. No. 5,228,861, "High Density Electrical Connector System", uses an array of "X" shaped contact elements on an insulating sheet, in which the ends of the arms of the "X" are bent up or down to make contact with the contact pads. PA0 Crotzer, U.S. Pat. No. 5,599,193, "Resilient Electrical Interconnect" and Crotzer, et. al, U.S. Pat. No. 5,949,029 describe the conductive polymer button based interposer system sold by Thomas & Betts Corp, of Memphis, Tenn., under the name "Metallized Particle Interconnect" (WPI). Being elastomer based, MPI suffers from relaxation of contact force over time, needs a high insertion force to offset deficiencies in planarity of the circuits to be connected, and, being based on silver flakes, is of relatively high cost. In addition, this connector is known to be very susceptible to intermittent opens during mechanical vibration tests. PA0 In an article entitled, "Button Contacts for Liquid Nitrogen Applications", Almquist described a "Fuzz Button" connector system (See Proceedings of the 1989 Electronic Components and Technology Conference, 1989, p. 88; IEEE no. 0569-5503/89/0088): "The `Fuzz Button` is a cylinder of gold plated wire, in most cases copper/two percent silver, fifty microns in diameter, formed into sizes to fit the product application. The buttons are inserted into 0.5 mm holes in an 0.75 mm epoxy glass carrier." PA0 Cinch Connectors, a division of Labinal Components and Systems, Inc., of Lombard, Ill., markets a "fuzz button" connector system, under the trademark "CIN::APSE". Being almost solid, it suffers from a high insertion force, and fabrication cost. The cost is typically 5-10 times higher than what would be desirable at present. PA0 Lopergolo, U.S. Pat. No. 5,800,184, "High Density Electrical Interconnect Apparatus", uses parallel metal elements to make the contact between the contact pads in the array. The metal elements may be rods, or might have bent wire parts at the ends. PA0 Ayala-Equillin, et. al, U.S. Pat. No. 5,441,690, "Process of Making Pinless Connector", and Shih, et. al, U.S. Pat. No. 5,810,607, "Interconnector with Contact Pins Having Enhanced Durability", both teach design and fabrication of inclined metal wires through a polymer film. Being continuous, the film is rigid and needs a high insertion force, the film is also very expensive to fabricate as it includes such individual steps as wire bonding. This system is also described in a paper by inventor Shih and others entitled "A Novel Elastomeric Connector for Packaging Interconnections, Testing and Burn-in Applications", Proceedings of the 1995 Electronic Components and Technology Conference, 1995, p. 126; IEEE no. 0569-5503/95/0000-0126. PA0 Bradley, et. al, U.S. Pat. No. 5,232,372, "Land Grid Array Connector and Method of Manufacture", uses separate coil springs set on their sides on rod-like carriers as the connection elements. PA0 Carey, U.S. Pat. No. 5,101,553, "Method of Making a Metal-on-Elastomer Pressure Contact Connector", produces an array of semicircular contact wires in an elastomeric interposer sheet by embedding coils of wire lengthwise in rows along the sheet, then cutting off the loops of wire which protrude above and below the elastomer. The result is rows of contact wires, spaced in one dimension by the coil spacing and in the other dimension by the thickness of the coil and the spacing between coils. The wire of necessity has to be rather thick, and controlling the position of the contacts is difficult, since at any point the position of the wire depends on where on the coil the wire happens to be when the loops are cut. PA0 Researchers at Nitto Denko Corporation, reported on a very high density anisotropic film, where metal wires are mounted perpendicularly through a polymer film ("Development of Novel Anisotropic Conductive Film (ACF)", Miho Yamaguchi, Fumiteru Asai, Fuyuki Eriguchi, Yuji Hotta Proceedings of the 1999 Electronic Components and Technology Conference, Jun. 1-4, 1999; San Diego, Calif.; p.360). As with the Shih, et. al interposer, this design necessitates very high insertion force (about 2000 lbs for a square inch module). Therefore, the design is suitable only for some square millimeter arrays, for example, chips used in wrist watches.
Typically the anisotropic adhesives described in the previous art do not take an advantage of the synergistic elastic properperties of metal-polymer composites. Thus, for example, in the anisotropically conductive film described by Yamaguchi et al., the conducting straight wires are merely glued together by the polymer, and therefore the elastic properties of the system are very closely 100% determined by the conductive wires alone. The same is true also for the anisotropically conductive film described in U.S. Pat. No. 5,101,553.
On the other hand, Ayala-Equillin et al., and Shih et al. describe an anisotropically conductive film with inclined straight wires. Because the wires are relatively rigid in comparison to polymer, they merely tilt as solid bodies during compression of the composite film, and therefore the deformation resistance in this case is determined almost entirely by the polymer matrix alone. These examples should be contrasted to the teachings of present invention, in which both the conductive structures and the elastomeric matrix deform elastically, such that there exist strong elastic interactions between the deforming bodies.
As a summary, the LGA interposers of the prior art suffer from at least 2 or more of the drawbacks:
High manufacturing cost (for density and I/O) PA1 High insertion force for dense or large arrays PA1 Difficulty of reaching high I/O at high density PA1 High profile (a drawback for high frequency and laptop form factor) PA1 Long term reliability is a concern with designs using an elastomer PA1 Adding discrete components, such as decoupling capacitors, is difficult when the insertion force is high, as this necessitates large clamp structures obstructing large areas of printed circuit board. PA1 Adding shielding for noise and impedance control is difficult.